An optical fiber, particularly the one composed of a silica has been used at the ultraviolet region, not to mention at the visible light region. As the core material, used are silica glasses containing OH group but substantially free of fluorine, which may or may not contain chlorine. Said core material is mainly characterized by OH group intentionally contained therein, and is advantageous in that its output is great and initial characteristic is superior. On the other hand, its ultraviolet deterioration resistance characteristic, namely, resistance to the deterioration of core material caused by ultraviolet rays, is not, in general, entirely superior, and specifically said characteristic is insufficient at the wavelength of 215 nm.
In the meantime, silica glass core materials for optical fibers which have been used at the visible light region have received increasing demand from the field involving exposure to radiation, such as for nuclear power, and resistance to radiation has recently been strongly demanded of the optical fiber for use in such fields.
In view of the background as described, the present inventors developed a new optical fiber to be used at the visible light region, specifically an optical fiber composed of a silica glass core made of a core material intentionally containing fluorine, but substantially free of chlorine and OH group. This core material exhibits extremely superior resistance to radiation at the visible light region, whereas when used at the ultraviolet region, shows small initial characteristic and small output: substantially it is not practical for use at 250 nm or below.